1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for processing an image of a fundus of the eye.
2. Description of the Related Art
Examination of a fundus of a subject's eye is widely accepted as important in the early diagnosis of diseases which are likely to cause lifestyle-related diseases and blindness. A scanning laser ophthalmoscope (SLO), which is an ophthalmologic apparatus that uses the technique of confocal laser scanning microscopy, performs raster scanning of a fundus of the eye using laser as a measuring beam and promptly obtains high-resolution planar images from the intensity of the return beam. In recent years, an adaptive optics SLO including an adaptive optical system has been developed and is used for acquiring planar images of high lateral resolution. The adaptive optical system measures an aberration of a subject's eye by a wavefront sensor in real time and corrects the aberration of the measuring beam and the return beam that occurs at the subject's eye by a wavefront correcting device. Further, attempts are being made to extract a portion that corresponds to photoreceptor cells of a retina using the obtained planar image of retina and make a diagnosis of a disease or evaluate drug response by analyzing the density or distribution of the portion thereof.
An ophthalmologic imaging apparatus which visualizes the photoreceptor cells by acquiring a planar image of a retina using the adaptive optics SLO and automatically extracting a portion corresponding to the photoreceptor cells is discussed in Kaccie Y. Li and Austin Roorda, “Automated identification of cone photoreceptors in adaptive optics retinal images” J. Opt. Soc. Am. A, May 2007, Vol. 24, No. 5, 1358. This document discusses a technique for automated detection of the photoreceptor cells using an ophthalmologic imaging apparatus. According to this technique, a planar image of a retina with high lateral resolution is acquired by preprocessing the acquired planar image, in other words, removing high frequency components from the planar image using periodicity of the arrangement of the photoreceptor cells visualized in the image. Further, Voronoi analysis of the distribution of the photoreceptor cells is performed using the density of the photoreceptor cells and the distance between such cells measured from the result of the detection of the photoreceptor cells.
In order to accurately analyze the photoreceptor cells, it is important to use an image of high image quality. As one method for obtaining a high quality image, there is known a method that removes random noise by continuously capturing a plurality of images of the same portion and superimposing the captured images. Generally, before the superimposing processing is performed, an image that can be used as a reference (hereinafter referred to as a reference image) is selected from a plurality of images. Then, the superimposing processing is performed after each of the images is subjected to the alignment processing with respect to the reference image.
It is known that a random motion referred to as an involuntary eye movement is constantly seen with a subject's eye. Thus, even if imaging of the same portion is continuously performed, the imaging position is actually not fixed. Under such circumstances, Japanese Patent Application Laid-Open No. 2008-293185 discusses a method useful for accurately performing positional alignment of a plurality of tomographic images using a reference image.
The involuntary eye movement is categorized into movements such as tremor, flick, drift, and saccade. Tremor is a very small high frequency oscillation of a few μm, flick is a low frequency oscillation of a few 10 μm, drift is a constant flow movement, and saccade is an extremely fast movement.
In performing superimposing processing of images of a fundus of an eye, it is important not to select the reference image from images captured during a large magnitude of movement such as a flick, and to select a group of images captured during a tremor, which occurs between flicks. Thus, there is a need for easily determining the presence/absence of misregistration of the photoreceptor cell image of a fundus of an eye.